1. Field of the Invention
The invention relates to monomeric or oligomeric materials in which polymerization-induced shrinkage is desirably avoided. In particular, the invention relates to holographic recording media formed from such materials.
2. Discussion of the Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, so-called page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Page-wise systems involve the storage and readout of a representation, e.g., a page, of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index in a storage medium. Holographic systems are discussed generally in D. Psaltis et al., "Holographic Memories," Scientific American, November 1995, the disclosure of which is hereby incorporated by reference. One method of holographic storage is phase correlation multiplex holography, which is described in U.S. Pat. No. 5,719,691 issued Feb. 17, 1998, the disclosure of which is hereby incorporated by reference. In one embodiment of phase correlation multiplex holography, a reference light beam is passed through a phase mask, and intersected in the recording medium with a signal beam that has passed through an array representing data, thereby forming a hologram in the medium. The spatial relation of the phase mask and the reference beam is adjusted for each successive page of data, thereby modulating the phase of the reference beam and allowing the data to be stored at overlapping areas in the medium. The data is later reconstructed by passing a reference beam through the original storage location with the same phase modulation used during data storage.
FIG. 1 illustrates the basic components of a holographic system 10. System 10 contains a modulating device 12, a photorecording medium 14, and a sensor 16. Modulating device 12 is any device capable of optically representing data in two-dimensions. Device 12 is typically a spatial light modulator that is attached to an encoding unit which encodes data onto the modulator. Based on the encoding, device 12 selectively passes or blocks portions of a signal beam 20 passing through device 12. In this manner, beam 20 is encoded with a data image. The image is stored by interfering the encoded signal beam 20 with a reference beam 22 at a location on or within photorecording medium 14. The interference creates an interference pattern (or hologram) that is captured within medium 14 as a pattern of, for example, varying refractive index. It is possible for more than one holographic image to be stored at a single location and/or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 22, depending on the particular reference beam employed. Signal beam 20 typically passes through lens 30 before being intersected with reference beam 22 in the medium 14. It is possible for reference beam 22 to pass through lens 32 before this intersection. Once data is stored in medium 14, it is possible to retrieve the data by intersecting reference beam 22 with medium 14 at the same location and at the same angle, wavelength, or phase (depending on the multiplexing scheme used) at which reference beam 22 was directed during storage of the data. The reconstructed data passes through lens 34 and is detected by sensor 16. Sensor 16 is, for example, a charged coupled device or an active pixel sensor. Sensor 16 typically is attached to a unit that decodes the data.
The capabilities of such holographic storage systems are limited in part by the storage media. Iron-doped lithium niobate has been used as a storage medium for research purposes for many years. However, lithium niobate is expensive, is relatively poor in sensitivity (1 J/cm.sup.2), has relatively low index contrast (An of about 10.sup.-4), and exhibits destructive read-out (i.e., images are destroyed upon reading). Alternatives have therefore been sought, particularly in the area of photosensitive polymer films. See, e.g., W. K. Smothers et al., "Photopolymers for Holography," SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The material described in this article contains a photoimageable material system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable material system is located within an organic polymer host matrix that is substantially inert to the exposure light. During writing of information into the material by exposure to radiation in selected areas, the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by induced polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient create a refractive index change, forming the hologram representing the data.
Most holographic systems of this type are based on photopolymerization of free-radical photosensitive monomers such as acrylate esters. See, for example, U.S. patent application Ser. No. 08/698,142 (our reference Colvin-Harris-Katz-Schilling 1-2-16-10), the disclosure of which is hereby incorporated by reference. A variety of such photosensitive monomers are commercially available. While media based on such monomers provide useful results, it is possible for such media to encounter several limitations. One typical problem is shrinkage introduced into a recording medium due to the polymerization of the photosensitive monomer. Specifically, because polymerized monomers occupy a smaller volume than individual monomers, each step of writing data into a recording medium introduces localized shrinkage. The combined effect of the localized shrinkage increases the difficulty in accurately retrieving the stored data.
One way to address this shrinkage problem is to adjust the optics of a holographic storage system based on the shrinkage, e.g., rotating the orientation of the medium based on the extent of shrinkage, as discussed in Zhao et al., "Shrinkage-corrected volume holograms based on photopolymeric phase media for surface-normal optical interconnects," Appl. Phys. Lett. 71 (11), Sep. 15, 1997, at 1464. Such a method is useful for some types of holographic storage, but is not suitable for data storage applications where images with a well-defined angular bandwidth are recorded.
Some photosensitive monomers were developed which exhibited less shrinkage than conventionally-used acrylate monomers. These monomers not only formed holograms by polymerization (as discussed above), but also exhibited ring-opening during such polymerization. (See, e.g., Waldman et al., "Cationic Ring-Opening Photopolymerizatin Methods for Volume Hologram Recording," SPIE Vol. 2689, 1996, at 127.) Waldman et al. used epoxy polymerization to form holograms, and, because epoxy polymerization involves opening of epoxide monomer rings, the polymerization exhibits about half as much shrinkage as acrylate polymerizations.
It has further been proposed that spiro-orthoesters and spiro-orthocarbonates, so-called expanding monomers, be added to epoxy polymerization systems or employed as the sole polymerizing monomers. (See, e.g., Expanding Monomers: Synthesis. Characterization, and Applications (R. K. Sadhir and R. M. Luck, eds., 1992) 1-25, 237-260; T. Takata and T. Endo, "Recent Advances in the Development of Expanding Monomers: Synthesis, Polymerization and Volume Change," Prog. Polym. Sci., Vol. 18, 1993, 839-870.) Such spiro compounds have been reported to exhibit relatively small shrinkage, or even expansion, upon polymerization due to opening of two rings per polymerized monomer functional group. (A monomer functional group is the group or groups on a photoactive monomer that are the reaction sites for polymerization, e.g., during the holographic writing process.) However, the shrinkage compensating ability of the spiro compounds is not as great as has been claimed or desired, primarily because the measured results are due, at least in part, to a phase change. (See C. Bolln et al., "Synthesis and Photoinitiated Cationic Polymerization of 2-methylene-7-phenyl-1,4,6,9-tetraoxaspiro[4,4]nonane," Macromolecules, Vol. 29, 1996, 3111-3116.) Specifically, solid forms of the spiro compounds are used in preparing the polymerizable material, and when the solid melts expansion occurs due to the phase change. Also, the rates of ring-opening and accompanying fragmentation side reactions are difficult to tune, and the reactions generally require relatively high temperatures (&gt;40.degree. C.). Thus, the ring-opening chemistries reported to date have not provided a reliable route to holographic media with near-zero dimensional change upon recording.
An improved holographic recording media is therefore desired which exhibits improved compensation for polymerization-induced shrinkage.